The early evolution of diasome molluscs

Abstract Bivalves have a wide distribution in the Lower and to a lesser extent Middle Cambrian rocks, but they have not yet been certainly identified in the Upper Cambrian. Recent discoveries have significantly increased our knowledge of Lower Ordovician bivalve faunas and their explosive radiation from the Early Ordovician apparently coincides with the evolution of the feeding gill. Early Ordovician faunas were confined to the siliciclastic facies of Gondwanan shelf seas; most genera were clearly latitudinally constrained, but others apparently migrated over wide latitudes. By the Mid-Ordovician, bivalves had begun to escape the confines of Gondwana and marked latitudinal differences in the composition of the faunas became apparent, with pteriomorphians showing clear preference for low latitudes, whilst heteroconchs clearly preferred median to high latitudes; surprisingly, nuculoids were both most diverse in terms of species and most abundant as a percentage of individuals within the bivalve populations at low latitudes. It was in the Late Ordovician that bivalves colonized the low-latitude carbonate platforms of Laurentia and Baltica, leading to a second major diversification particularly within the pteriomorphian bivalves, which developed semi-infaunal and epifaunal habits; they became the dominant low-latitude bivalve group. The latest-Ordovician eustatic regression which exposed the low-latitude carbonate platforms resulted in a major reduction in the epifaunal and semi-infaunal bivalves involving extinction of many genera, including the only Ordovician boring bivalves.

Diversification and biogeography of bivalves during the Ordovician Period

In the Ordovician Northern Belt of the Southern Uplands, basal volcanics (Arenig) are followed by cherts (Llanvirn-?Llandeilo), then by graptolite shales (Llandelio-Lower Caradoc), and finally by Caradoc greywackes Within the greywackes (Kirkcolm Formation) are a number of occurrences of fossiliferous conglomerate and overlying mudstone; these can be traced along-strike for some 30 km



The conglomerates, and especially the mudstones, yield rich mid- Caradoc shelly assemblages; brachiopods (20 spp)/ trilobites (14 spp), gastropods, bivalves, bryozoans, and the first known Palaeozoic scleractiniamorph coral



These occurrences are interpreted as mass-flow deposits derived by downslope movement from a now-vanished shelf to the North, and may belong either to a single gigantic debris flow event, or to a series of smaller, but roughly contemporaneous flows



Strong faunal similarities to faunas at Girvan (western Scotland), and Tyrone (Northern Ireland), lying North of the Southern Upland Fault may suggest sinistral strike-slip movement of no more than a few hundred kilometres

Ordovician faunas in mass-flow deposits, Southern Scotland

The Treptoceras duseri shale unit within the Waynesville Formation of Late Ordovician (early Richmondian) age in southwest Ohio and the equivalent Trilobite shale unit in the same formation exposed in adjacent portions of Indiana represent an Ordovician shallow marine mud-bottom epeiric sea facies. These fine-grained elastics contain a moderately diverse mollusk-trilobite assemblage dominated by vagrant epifaunal detritus-feeding calymenid and asaphid trilobites, large endobyssate and infaunal filter-feeding pelecypods, and nektonic nautiloids. Articulate brachiopods, ectoprocts, and pelmatozoan echinoderms form only minor elements of this fauna.This mollusk-trilobite assemblage was common in Late Ordovician shallow marine clastic environments where mobility was an asset and there was an abundance of oxygen and food resources. Such assemblages are characteristic of the Lorraine Fauna of Late Ordovician (Edenian to Richmondian) age that occurs from the Ohio Valley north and east into New York, Ontario, Quebec, and Ireland. These early Paleozoic mud-bottom assemblages were considerably modified by the Late Ordovician extinction event and were replaced in the Silurian and Devonian by distinctly different assemblages dominated by large epifaunal strophomenid and spiriferid brachiopods, crinoids, and phacopid trilobites.

The paleoecology of a late ordovician shale unit from southwest ohio and southeastern indiana

https://onlinelibrary.wiley.com/doi/pdf/10.1111/1475-4983.00081

S. P. Tunnicliff

Papers

A revision of Late Ordovician bivalves from Pomeroy, Co Tyrone, Ireland